1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventional image forming apparatus such as copiers and printers which use an electrophotographic method include a type which uses an intermediate transfer belt. An image forming apparatus of an intermediate transfer belt type forms a full-color image through a primary transfer process and secondary transfer process. In the primary transfer process, a toner image formed on a surface of an electrophotographic photosensitive body is transferred onto the intermediate transfer belt. This process is repeated for toner images of plural colors, thereby forming toner images of the plural colors on the surface of the intermediate transfer belt. In the secondary transfer process, the toner images of the plural colors are transferred altogether onto a surface of a transfer material such as paper. The toner images transferred onto the surface of the transfer material are fixed by a fixing unit. This produces a full-color image.
A roller-typed, blade-typed, or brush-typed transfer member is used as primary transfer members of the image forming apparatus. The transfer member is used for contacting a back surface of the intermediate transfer belt at a position opposite the photosensitive body, with a primary transfer voltage applied thereto. Among others, the brush-typed transfer member is made up of a conductive filament group, and each and every one of the filaments can independently come into contact with the back surface of the intermediate transfer belt. This remedies uneven contact caused when a roller type or blade type transfer member is used and enables more uniform contact with the back surface of the intermediate transfer belt. This properly reduces image defects such as density unevenness occurring in the primary transfer process.
As a method for coming into contact with the back surface of a belt of a brush-typed transfer member, Japanese Patent Application Laid-Open No. 2001-134115 discloses a method for coming into contact with the belt by tilting the transfer member such that filaments will come into contact with the back surface of the intermediate transfer belt by being inclined downstream in a moving direction of the belt. With this contact method, an inclination direction of the filaments can be limited to a direction toward a downstream side of the moving direction of the belt.
However, in the moving direction of the belt, the upstream side of the brush-typed transfer member is sometimes inclined randomly. It is necessary that an upstream end of the brush-typed transfer member is placed at a predetermined position relative to an upstream end of a contact area between a photosensitive drum and the intermediate transfer belt. If the position of the upstream end of the brush-typed transfer member is displaced from the contact area, there is a fear that an electric discharge might occur upstream of the contact area, degrading primary transfer property.
In particular, with the brush-typed transfer member, plural conductive filaments located at an upstream end are often randomly oriented. Consequently, even if the brush-typed transfer member is positioned accurately with respect to the contact area, positions at which the conductive filaments at the upstream end come into contact with the intermediate transfer belt might vary with respect to a direction orthogonal to the moving direction of the intermediate transfer belt, resulting in degradation of primary transfer property.
Note that, this problem occurs not only in the image forming apparatus which use an intermediate transfer belt as a transfer belt, but also in image forming apparatus which use a transport belt adapted to transport a transfer material, as a transfer belt.